1. Field of the Invention
This invention relates to a substrate with a transparent conductive film and an organic electroluminescence (hereinafter referred to as xe2x80x9cELxe2x80x9d) device using the same, and more particularly to a substrate with a transparent conductive film, which is used as an electrode (anode) of an EL display device, and an organic EL device having an organic multilayer film laminated on a surface of the transparent conductive film.
2. Prior Art
The organic EL device is a display device which has an organic multilayer film comprised of a hole transport layer, a light-emitting layer, and an electron transport layer, interposed between an anode and a cathode, for carrying out an electric charge injection/recombination-based light-emitting operation. Recently, the organic EL device has been actively researched and developed in view of its advantageous features as a display device, i.e. a lower driving voltage and a wide range of possible luminescent colors which can be realized due to the variety of organic materials available therefor.
In an organic EL device of this kind, holes injected from the anode and electrons injected from the cathode travel through the hole transport layer and the electron transport layer to the light-emitting layer where the holes and the electrons are recombined to perform a light-emitting operation. As the cathode, a metal material, such as aluminum (AL) is employed, and as the anode, indium tin oxide (hereinafter referred to as xe2x80x9cITOxe2x80x9d) is employed, which has an excellent transparency and a low electrical resistance.
In the organic EL device of the above-mentioned kind, when holes are injected from the anode into the light-emitting layer, the holes move from the anode to the hole transport layer across an energy barrier between the anode and the hole transport layer. The work function of an ordinary ITO film, however, is by far smaller than the ionization potential Ip of the hole transport layer (the ionization potential Ip of the hole transport layer is normally 5.5 to 5.6 eV whereas the work function of the surface of the ITO film formed by a sputtering method is 4.2 to 4.7 eV), so that the balance of injection into the light-emitting layer between injected holes and injected electrons is lost, resulting an increase in the driving voltage. Therefore, to attain a reduced driving voltage, it is necessary to enhance the efficiency of injection of holes into the hole transport layer, and to this end, the energy barrier between the anode and the hole transport layer is required to be minimized.
Further, in the organic EL device, if the transparent conductive film as the anode has a large surface roughness (difference in height between protruding portions and recessed portions), a high electric field can be concentratedly applied to protruding portions of the surface. This causes a slight electric discharge to occur at the protruding portions, and therefore makes the device prone to a breakdown to form dark points from which light is no longer emitted. In short, the durability of the organic EL device is degraded. Therefore, the transparent conductive film is required to have a minimized surface roughness, i.e. an excellent smoothness.
To meet the above requirements, there has been proposed a technique by Japanese Laid-Open Patent Publication (Kokai) No. 8-167479 (hereinafter referred to as xe2x80x9cthe first prior artxe2x80x9d), which carries out annealing on an ITO film formed as a transparent conductive film to obtain a smooth surface on the ITO film, and then subjects the resulting surface to further annealing or plasma processing, thereby increasing the work function of the ITO film to reduce the energy barrier between the anode and the hole transport layer.
More specifically, in the first prior art, after depositing amorphous particles formed of ITO onto the substrate, annealing is carried out within a temperature range of 100 to 500xc2x0 C. under a non-oxidizing atmosphere, thereby causing crystal growth of the particles such that the ITO film has a surface roughness of 10 nm or less. Further, annealing is effected on the surface within a temperature range of 100 to 500xc2x0 C. under an oxidizing atmosphere or a plasma is irradiated onto the surface, to make the work function of the surface of the ITO film larger than that of an ordinary ITO film, whereby the energy barrier between the anode and the hole transport layer of the ITO film is reduced.
In the first prior art, however, although the work function of the ITO film can be increased and the surface roughness can be reduced to a level of 10 nm or less to obtain a good smoothness, the film exhibits a high specific resistance of 2xc3x9710xe2x88x924 xcexa9xc2x7cm or higher, since the ITO film is formed by the sputtering method.
More specifically, differently from a voltage driven device, such as a crystal display device, the organic EL device, which is a current driven device, is susceptible resistance in power consumption and display quality to the wiring. To avoid this, it is necessary to reduce the resistance of the transparent conductive film (ITO film) as the anode, and to this end, it is required to minimize the specific resistance of the ITO film. However, the ITO film according to the above first prior art, which is formed by the conventional sputtering method, has a high specific resistance, which not only causes a large power loss of an organic EL device using the ITO film owing to a high wiring resistance, resulting in an increase in the power consumption, but also degrades display quality, which makes the ITO film difficult to be applied to today""s EL devices of which capability of displaying high-precision images is demanded.
Further, the above first prior art is disadvantageous in that the film manufacturing process is complicated since a post treatment, such as annealing, is required.
As another conventional technique, there has been proposed a method of adding a metal oxide having a high work function, such as ruthenium oxide, molybdenum oxide or vanadium oxide, to ITO (Japanese Laid-Open Patent Publication (Kokai) No. 2000-72526; hereinafter referred to as xe2x80x9cthe second prior artxe2x80x9d).
The second prior art adds the above-mentioned metal oxide which has a higher work function than that of the ordinary ITO, to ITO to thereby increase the work function of the surface of an ITO film as a transparent conductive film. This enables reduction of the energy barrier between the anode and the hole transport layer.
The second prior art, however, has the disadvantage of increased manufacturing costs due to the use of a special and expensive metal oxide, such as ruthenium oxide or molybdenum oxide, which is added to the ITO. Moreover, similarly to the first prior art, the second prior art employs the sputtering method in forming the ITO film, which therefore only has a specific resistance of 7xc3x9710xe2x88x924 xcexa9xc2x7cm or higher. Similarly to the first prior art, this results in increased power consumption and degraded display quality. This makes the ITO film produced by the second prior art difficult to be put to practical use in a high-precision EL display device.
Still another conventional technique has also been proposed which forms an ITO film into a bilayer structure, whereby one ITO film at an interface side in contact with a hole transport layer has an increased work function (Japanese Laid-Open Patent Publication (Kokai) No. 2000-68073; hereinafter referred to as xe2x80x9cthe third prior artxe2x80x9d).
The third prior art attempts to obtain an improved hole injection characteristic, by changing the oxygen partial pressure during a film forming process for forming an ITO film by the sputtering method, so that the ionization potential Ip, i.e. the work function of the interface portion of the ITO film is made closer to the ionization potential Ip of the hole transport layer.
More specifically, the third prior art changes the oxygen partial pressure during the film forming process, thereby forming an ITO film having a bilayer structure comprised of two layers having respective different ionization potentials Ip. However, similarly to the first prior art and the second prior art, the third prior art as well forms an ITO film by the sputtering method, and hence the ITO film has a high specific resistance, resulting in increased power consumption and degraded display quality. Therefore, this ITO film also suffers from the problem that it is difficult to put the same to practical use in a high-precision EL display device.
It is an object of the invention to provide a substrate with a transparent conductive film, which has a high work function and an excellent surface smoothness as well as a reduced specific resistance to thereby ensure a reduced power consumption and enhanced display quality, and an organic EL device using the same.
It is desirable that a transparent conductive film used in an organic EL device as an anode should have characteristics of a high work function, an excellent surface smoothness, and a decreased specific resistance for reducing power consumption and enhancing display quality.
However, if the sputtering method is employed to form an ITO film as in the prior art techniques described above, it is required to heat the glass substrate to a temperature as high as 300xc2x0 C. or higher to obtain an ITO film having a desired low specific resistance. Moreover, in this case, to ensure excellent reproducibility of the film formation, it is necessary to form the ITO film at a very low film-forming speed, which results in poor mass productivity. Further, the ITO film formed as above by heating the glass substrate to a temperature as high as 300xc2x0 C. or higher has a surface roughness of 30 nm or larger.
On the other hand, in the case of forming a transparent conductive film on a transparent substrate by the conventional sputtering method, so far as the work function and the surface roughness are concerned, it is possible to achieve values of these characteristics satisfactory to some extent. However, it is difficult to mass-produce transparent conductive films having a low specific resistance of 1.6xc3x9710xe2x88x924 xcexa9xc2x7cm or less on an industrial scale, and hence difficult to reduce power consumption and enhance display quality.
In view of the above circumstances, the present inventors have made intensive studies and reached a finding that if a sintered compact of ITO as a transparent conductive material with a tin oxide (SnO2) content of 4 to 6 wt % is employed as a material to be vaporized, and a transparent conductive film is formed on a transparent substrate by an ion plating method, it is possible to obtain a substrate with a transparent conductive film, for use with an organic EL device, which satisfies all the requirements of characteristics, such as work function, smoothness, and specific resistance.
The present invention is based on the above finding, and provides a substrate with a transparent conductive film comprising a transparent substrate, a transparent conductive film formed on a surface of the transparent substrate, wherein the transparent conductive film has a work function of 4.9 to 5.5 eV, a surface roughness of 1 to 10 nm, and a specific resistance of 1.6xc3x9710xe2x88x924 xcexa9xc2x7cm or less. It is preferable that the transparent conductive film is formed on the surface of the transparent substrate by an ion plating method by using indium tin oxide which is a mixture of tin oxide and indium oxide as a material to be vaporized, and the indium tin oxide has a tin oxide content of 4 to 6 wt %.
When the above substrate with the transparent conductive film is used as a substrate in an organic EL device, it is possible to secure an increased efficiency of injection of holes into a hole transport layer (hole injection efficiency), an excellent smoothness, and further a reduced wiring resistance. Moreover, the substrate with the transparent conductive film, which is also excellent in productivity, can be obtained without carrying out a complicated post treatment.
Further, by laminating an organic multilayer film onto a surface of the transparent conductive film which has a high work function, an excellent smoothness, and a low specific resistance, it is possible to obtain an organic EL device which is reduced in driving voltage, and enhanced in durability and display quality with reduced power consumption.
That is, the organic EL device according to the present invention is characterized in that a multilayer film including a hole transport layer formed of an organic material is laminated on a surface of the transparent conductive film of the substrate with the transparent conductive film.
According to this organic EL device, the multilayer film including the hole transport layer formed of an organic material is laminated on the surface of the transparent conductive film, so that it is possible to decrease an energy barrier between the hole transport layer and the transparent conductive film, and reduce the driving voltage to 20 V or less. At the same time, the excellent smoothness of the transparent conductive film can improve durability, and the low specific resistance of the same reduces power loss, thereby making it possible to obtain a high display quality at a reduced power consumption. That is, all the requirements of the characteristics for the organic EL device can be satisfied.
Further, the ionization potential Ip of the hole transport layer is generally 5.5 to 5.6 eV. Therefore, when an organic EL device is produced by using the substrate with the transparent conductive film, the energy barrier between the hole transport layer and the transparent conductive film is reduced to 0.7 eV or less, thereby making it possible to increase the hole injection efficiency of injection of holes from the anode into the hole transport layer. This enables the reduction of the driving voltage.
Therefore, it is preferred that the energy barrier between the transparent conductive film and the hole transport layer is equal to or smaller than 0.7 eV.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.